Multi-channel optical device

ABSTRACT

The multi-channel optical device includes a demultiplexer in a laser cavity. The demultiplexer is configured to demultiplex a multi-channel light beam into a plurality of channels. The device also includes a plurality of ports. Each channel exits the laser cavity through a different one of the ports.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patentapplication Ser. No. 60/872,375, filed on Dec. 1, 2006, entitled“Multi-Channel Optical Device” and incorporated herein in its entirety.

FIELD

The present invention relates to optical devices and more particularlyto devices for creating a light beam that includes multiple channels.

BACKGROUND

Optical communication systems employ waveguides to carry opticalchannels. The waveguides preferably carry a plurality of opticalchannels in order to increase the capacity of the system. These channelsare generated by lasers. Fabry-perot (FP) lasers emit a broad range ofwavelengths but the emission spectrum is not easily controlled. Thespectrum changes with temperature and current and is not capable of highspeed transmission or over long spans. In response, distributed feedback(DFB) lasers were generated. While DFB lasers are able to emit over anarrow spectrum they are only capable of generating a single wavelengthchannel. As a result, a plurality of DFB lasers are often employed togenerate the desired number of channel. However, DFB lasers aresubstantially more expensive that FP lasers. As a result, using multipleDFB lasers can cause an undesirable increase in the cost of the system.As a result, there is a need for an economical optical device that canproduce a plurality of channels such that each channel has a narrowrange of wavelengths.

SUMMARY

A multi-channel optical device includes a demultiplexer in a lasercavity. The demultiplexer is configured to demultiplex a multi-channellight beam into a plurality of channels. The device also includes aplurality of ports. Each channel exits the laser cavity through adifferent one of the ports.

Another embodiment of the multi-channel optical device includes ademultiplexer in a laser cavity. The demultiplexer is configured todemultiplex a multi-channel light beam into a plurality of channels. Achannel waveguide is configured to receive one of the channels. Eachchannel waveguide is included in an optical coupler. Each opticalcoupler includes a coupled waveguide optically coupled with one of thechannel waveguides such that a portion of a channel traveling along thechannel waveguide enters the coupled waveguide. A plurality ofreflectors are each configured to reflect at least a portion of thechannel that entered one of the coupled waveguide back into the coupledchannel waveguide such that the reflected portion of the channel travelsthrough the channel waveguide back to the demultiplexer.

Another embodiment of the multi-channel optical device includes ademultiplexer in a laser cavity. The demultiplexer is configured todemultiplex a multi-channel light beam into a plurality of channels. Thedevice also includes a plurality of modulators. Each modulator isconfigured to modulate one of the channels after the channel has exitedthe laser cavity. Modulation of each channel results in a modulatedchannel.

Another embodiment of the multi-channel optical device includes ademultiplexer in a laser cavity. The demultiplexer is configured todemultiplex a multi-channel light beam into a plurality of channels. Thedevice also includes a multiplexer configured to multiplex the channelsafter the channels exit the laser cavity.

Another embodiment of the multi-channel optical device includes ademultiplexer in a laser cavity. The demultiplexer is configured todemultiplex a multi-channel light beam into a plurality of channels. Aplurality of channel waveguides are each configured to receive one ofthe channels. Each channel waveguide includes a port through which oneof the channels exits the laser cavity. Additionally, each channelwaveguide is included in an optical coupler. Each optical couplerincludes a coupled waveguide optically coupled with one of the channelwaveguides such that a portion of a channel traveling along the channelwaveguide enters the coupled waveguide. The device includes a pluralityof reflectors that are each configured to reflect a portion of thechannel that enters one of the coupled waveguides back onto the coupledchannel waveguide such that the reflected portion of the channel travelsthrough the channel waveguide back to the demultiplexer. A plurality ofmodulators are each configured to modulate a channel traveling along oneof the channel waveguides after the channel has passed through one ofthe optical couplers. Modulation of each channel resulting in amodulated channel. A multiplexer is configured to receive a modulatedchannel from each of the modulators and to multiplex the modulatedchannels into a beam having a plurality of modulated channels.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is schematic view of a multi-channel device.

FIG. 1B illustrates combination of channels from different multi-channeldevices.

FIG. 2 illustrates a multi-channel device constructed according to FIG.1A.

FIG. 3A through FIG. 3D illustrate the portion of a multi-channel devicehaving an interface between a cavity waveguide and a gain medium chip.FIG. 3A is a topview of the multi-channel device.

FIG. 3B is a cross section of the cavity waveguide shown in FIG. 3Ataken along the line labeled B.

FIG. 3C is a cross section of the multi-channel device shown in FIG. 3Ataken along a line extending between the brackets labeled C in FIG. 3A.

FIG. 3D is a cross section of the multi-channel device shown in FIG. 3Ataken along a line extending between the brackets labeled D in FIG. 3A.

FIG. 4A and FIG. 4B illustrate a portion of a multi-channel devicehaving an interface between an echelle grating, a cavity waveguide, andchannel waveguides. FIG. 4A is a topview of the multi-channel device.

FIG. 4B is a cross section of the multi-channel device taken along theline labeled B in FIG. 4A.

FIG. 5A through FIG. 5C illustrate a portion of a multi-channel devicehaving an optical coupler. FIG. 5A is a topview of the multi-channeldevice. The coupler includes a channel waveguide and a coupledwaveguide.

FIG. 5B is a cross section of the coupled waveguide and the channelwaveguide taken along the line labeled B in FIG. 5A.

FIG. 5C is a cross section of the coupled waveguide taken along a linebetween the brackets labeled C in FIG. 5A.

FIG. 6A through FIG. 6E illustrate a portion of a multi-channel devicehaving a mach-zehnder interferometer configured to operate as anintensity modulator. FIG. 6A is a topview of the mach-zehnderinterferometer. The mach-zehnder interferometer includes a channelwaveguide that branches into a first branch waveguide and a secondbranch waveguide. The first branch waveguide re-joins the second branchwaveguide at a modulated waveguide. A phase modulator is positionedalong the second branch waveguide.

FIG. 6B is a topview of a phase modulator that is suitable for use withthe mach-zehnder interferometer of FIG. 6A.

FIG. 6C is a cross section of the phase modulator shown in FIG. 6B takenalong the line labeled C in FIG. 6B.

FIG. 6D through FIG. 6E illustrate the effects of tuning the phasemodulator on the second branched waveguide.

FIG. 6F illustrates the phase modulator of FIG. 6B through FIG. 6Esubstituted for the intensity modulator illustrated in FIG. 6A.

FIG. 7 is a cross section of a waveguide on a silicon-on-insulatorwafer. The waveguide has a width labeled W. The width is the width ofthe ridge at the top of the ridge. The waveguide also has a thicknesslabeled H.

FIG. 8A is a topview of a portion of the multi-channel device having aninterface between an echelle grating, a cavity waveguide, and a channelwaveguides. The channel waveguides each include a taper.

FIG. 8B is a topview of a portion of a multi-channel device having amodulator positioned between waveguide tapers.

DESCRIPTION

A multi-channel device includes a demultiplexer in a laser cavity. Thedemultiplexer is configured to demultiplex a light beam having a broadrange of wavelengths into a plurality of channels that each has a rangeof wavelength. The presence of the demulitplexer in the laser cavityreduces the range of wavelengths that are lased to a range that issmaller than the range of wavelengths for the channel. The lasedwavelengths exit the laser cavity and serve as the output of the lasercavity. As a result, the multi-channel device can employ a single lasercavity and output a plurality of channels that each has a narrow rangeof wavelengths.

Each channel exits the laser cavity through a different port. As aresult, a plurality of channels can be produced from a single lasercavity. Additionally, the demultiplexer can control the range ofwavelengths in each channel. As a result, the multi-channel device canemploy a single laser cavity and still produce a plurality of channelswhere each channel has a narrow range of wavelengths.

A multi-channel device includes a first reflector and partial returndevices. As the channels travel back and forth between the firstreflector and the partial return devices the channels travel through thegain medium where they are amplified and lase. Since the wavelengthsoutside the channels have a high level of loss due to the demultiplexer,these wavelengths are not amplified and do not lase. The Gaussian natureof the demultiplexer together with the spatial and spectral competitionof the wavelengths in the channels for a limited gain results in lasingof a narrower range wavelengths than the demultiplexer selects. As aresult, the range of wavelengths that lase in each channel is actuallysmaller than the range of wavelengths included in the channel.

As a result, a plurality of channels can be produced from a single lasercavity. Additionally, the demultiplexer can control the range ofwavelengths in each channel. As a result, the multi-channel device canemploy a single laser cavity and still produce a plurality of channelswhere each channel has a narrow range of wavelengths.

The multi-channel device can include a plurality of modulators. Eachmodulator receives one of the channels after the channel exits the lasercavity and modulates the channel so as to produce a modulated channel.The modulated channels are received at a multiplexer that multiplexesthe modulated channels onto an output beam. As a result, themulti-channel device can employ a single laser cavity and output a beamhaving a plurality of modulated channels. The modulators allow thechannels to be modulated without directly modulating a laser. Since thelaser is not directly modulated, the length of the laser cavity does notsubstantially affect the performance of the multi-channel device.

FIG. 1A is a schematic diagram of a multi-channel device. Themulti-channel device includes a gain medium 10 and ademultiplexer/multiplexer (demultiplexer 12) optically positionedbetween a first reflector 14 and a plurality of partial return devices16. Suitable gain media include, but are not limited to, InP, InGaAsP,GaAs, and Silicon Based Amplifiers. Suitable demultiplexers include, butare not limited to, echelle gratings, AWG demultiplexers, transmissiongratings, reflection gratings and other dispersive elements. The firstreflector 14 is preferably highly reflective or even 100% reflective.Suitable first reflectors 14 include, but are not limited to, mirrors,reflective metals, partially or fully metal coated waveguide facets. Thepartial return devices 16 are configured to return a portion of a lightsignal along its original path and to permit another portion of thelight signal to travel along a different path. Suitable partial returndevices 16 include, but are not limited to, partially reflectivesurfaces, optical couplers where the coupled waveguide 44 has areflective facet, partially etch facets, and narrow etched gaps. As willbe described in more detail below, the first reflector 14 and thepartial return devices 16 define a laser cavity.

During operation of the multi-channel device, the gain medium 10receives energy from a power source 18. The energy received from thepower source can be optical or electrical. In response to receiving theenergy, the gain medium emits a light beam having over a broad range ofwavelength as defined by the material properties of the gain medium. Thebeam is received at a demultiplexer. The demultiplexer separates thebeam into a set of channels where each channel includes a range ofwavelengths. The channels are illustrated in FIG. 1A as λ₁, λ₂, λ₃, andλ₄. Wavelengths outside of the channels are blocked by the demultiplexerby virtue of experiencing a high level of loss caused by thedemultiplexer. As a result, the demultiplexer provides the laser cavitywith wavelength selectivity.

The channels λ₁, λ₂, λ₃, λ₄ are each received at a different one of thepartial return devices 16. Each partial return device 16 is configuredto return a first portion of a received channel to the demultiplexer 12.The demultiplexer 12 multiplexes the portion of the channels returnedfrom the partial return devices 16. Accordingly, the demultiplexer 12also operates as a multiplexer. The gain medium 10 receives themultiplexed channels from the demultiplexer 12. The first reflector 14can receive the returned channels from the gain medium 10. The returnedchannels are reflected off the first reflector 14 and back through thegain medium 10. Accordingly, the channels travel back and forth betweenthe first reflector 14 and the partial return device 16.

As the channels travel back and forth between the first reflector andthe partial return device the channels travel through the gain mediumwhere they are amplified and lase. Since the wavelengths outside thechannels have a high level of loss due to the demultiplexer, thesewavelengths are not amplified and do not lase. The distribution ofwavelengths in a channel together with the competition of thewavelengths in the channels for a limited gain results in lasing of anarrower range wavelengths than the demultiplexer selects. As a result,the range of wavelengths that lase in each channel is actually smallerthan the range of wavelengths included in the channel.

As noted above, the partial return devices 16 each return a firstportion of a channel to the demultiplexer 12. A second portion of eachchannel exits the laser cavity through a partial return device 16.Accordingly, the partial return devices 16 each include a port throughwhich the channels exit the laser cavity. Channel that exit the lasercavity serve as the output of the laser.

The partial return devices can be tunable. For instance, the partialreturn devices can be tuned such that the ratio of the first portion ofa channel to the second portion of a channel can be tuned. Accordingly,tuning of a partial return device can increase or decrease the power ofa particular channel output by the laser. As a result, the partialreturn devices can be tuned so as to balance the power of differentchannels.

The multi-channel device includes a plurality of modulators 20. Eachmodulator 20 receives the portion of a channel transmitted by a partialreturn device 16. The modulator 20 can be an intensity modulator such asmonolithically integrated silicon modulator or other type of modulatorshybridized into a silicon platform. The modulator could also be anintensity modulator that includes a phase modulator. For instance, themodulator could also be a phase modulator used within a Mach Zehnderstructure. The modulators 20 permit independent modulation of eachchannel. Additionally, the presence of the modulators 20 means that thelaser can be a continuous wave laser that does not need its ownmodulation. Since the laser does not need its own modulation, the lengthof the laser cavity does not substantially affect the output of themulti-channel device.

The modulators 20 each output a modulated channel, λ_(1m), λ_(2m),λ_(3m), λ_(4m). A multiplexer 22 receives the modulated channels andmultiplexes them to provide an output beam that contains each of themodulated channels, λ_(1m), λ_(2m), λ_(3m), λ_(4m). Suitablemultiplexers 22 include, but are not limited to, echelle gratings, AWGmultiplexers, transmission gratings, reflection gratings or otherdispersive elements.

The wavelengths included in the channels λ₁, λ₂, λ₃, λ₄ can be tuned.For instance, the demultiplexer 12 can be a tunable demultiplexer.Tuning the demultiplexer tunes the wavelength of each channel. Anexample of a suitable tunable AWG demultiplexers are presented in U.S.patent application Ser. No. 09/945,685, filed on Apr. 30, 2001, entitled“Tunable Filter,” and now U.S. Pat. No. 6,853,773 and in U.S. patentapplication Ser. No. 09/993,337, filed on Nov. 13, 2001, entitled“Optical Component Having a Light Distribution Component With an Indexof Refraction Tuner,” each of which is incorporated herein in itsentirety. The tuning principles disclosed in these applications can alsobe applied to other demultiplexers such as echelle gratings.

The multi-channel device can optionally optical attenuators positionedin the laser cavity. The optical attenuators 23 can be positioned suchthat each optical attenuator is configured to attenuate the intensity ofone of the channels. For instance, each optical attenuator can bepositioned optically between the demultiplexer 12 and the partial returndevice 16 for a channel to be attenuated by the optical attenuator. Oneor more of the optical attenuators can be tunable. In one example, allof the optical attenuators are tunable. For instance, one or more of theoptical attenuators can be a variable optical attenuator. Tunability ofthe optical attenutors permits balancing of the power of the differentchannels to compensate for loss or gain bias inside the laser cavity. Ingeneral, the channels having the highest and lowest wavelengths (λ₁ andλ₄ in FIG. 1) to see lower gains and higher losses. The attenuators canbe employed to increase the loss for the channels with the centralwavelengths (λ₂ and λ₃ in FIG. 1). Placement of the optical attenuatorsin the lasing cavity permits a redistribution of power to take place inthe laser cavity such that losses introduced to the channels with thecentral wavelengths causes a redistribution of gain medium power to thechannels having the highest and lowest wavelengths. As a result, ahigher average power can be achieved. The redistribution of power maynot be achieved when optical attenuators are positioned outside of thelaser cavity.

The multi-channel device can optionally include an optical amplifier 25configured to amplify the modulated optical signals after they aremultiplexed at the multiplexer 22. As a result, the optical amplifiercan concurrently amplify all of the channels.

Although the multi-channel device is illustrated as producing only fourchannels, the multi-channel device can be configured to produce morethan four channels or fewer than four channels. The modulated channelsfrom several different multi-channel devices can be multiplexed tofurther increase the number of channels. For instance, the modulatedchannels labeled λ_(1m) through λ_(4m) in FIG. 1B could originate from afirst gain medium 10 and/or a first laser cavity while the channelslabeled λ_(5m) through λ_(8m) in FIG. 1B could originate from a secondgain medium 10 and/or from a second laser cavity. The multiplexer 22multiplexes the channels from both gain media to form a beam havingchannels λ_(1m) through λ_(8m). Alternately, the multiplexer 22multiplexes the channels from different laser cavities to form a beamhaving channels λ_(1m) through λ_(8m). Additionally, more than onemultiplexer can be employed to multiplex channels from different lasercavities. For instance, cascaded multiplexers can be employed tomultiplex channels from different laser cavities. When one or moremultiplexers the multiplexer 22 multiplexes the channels from differentlaser cavities, the different laser cavities and the one or moremultiplexers can be included on the same multi-channel device.

FIG. 2 illustrates a layout of a multi-channel device according to FIG.1A. The illustrated multi-channel is suitable for use in conjunctionwith optical components. The multi-channel device employs a gain mediumchip 24 that includes the gain medium 10 of FIG. 1A. The gain mediumchip 24 has a reflecting surface 28 that serves as the first reflector14 of FIG. 1A. The opposing surface includes an anti-reflective coating29. The multi-channel device employs an electrical power source 18 topass a current through the gain medium 10 and generate the light beam.The beam exits the gain medium 10 through the surface with theanti-reflective coating 29 and enters a cavity waveguide 32.

The multi-channel device of FIG. 2 employs an echelle grating 34 as thedemultiplexer 12 of FIG. 1A. The echelle grating 34 includes a freespace region 36 and a reflecting surface 38. The beam from the cavitywaveguide 32 enters the free space region 36 of the echelle grating 34.The path of the light through the echelle grating 34 is illustrated asdashed lines in FIG. 2 in order to distinguish the light from otherfeatures of the multi-channel device. The beam travels through the freespace region 36 and is reflected off of the reflecting surface 38. Thedetails of the reflecting surface 38 are not shown in order to simplifythe illustration. However, the reflecting surface 38 of an echellegrating 34 includes a plurality of stepped reflecting surfaces. Thereflecting surface 38 causes light of different wavelengths to separateas they travel away from the reflecting surface 38. Accordingly, theechelle grating 34 demultiplexes the beam into the individual channelstraveling away from the reflecting surface 38. The channels are eachreceived on a channel waveguide 40.

The multi-channel device of FIG. 2 employs an optical coupler 42 as thepartial return device 16 of FIG. 1A. Each coupler 42 couples a channelwaveguide 40 with a coupled waveguide 44. The coupler 42 is constructedsuch that a portion of the channel traveling along a channel waveguide40 is coupled into the associated coupled waveguide 44. The coupledwaveguide 44 includes a reflecting device 46 that causes at least aportion of the channel to travel back along the coupled waveguide 44into the channel waveguide 40 and back to the demultiplexer 12 of FIG. 1and accordingly the gain medium 10 of FIG. 1. A suitable reflectingdevice 46 includes, but is not limited to, a partially or completelyreflecting surface at the end of the coupled waveguide 44.

The coupler 42 controls the portion of a channel returned to the gainmedium 10. For instance, increasing the portion of the channel coupledinto the coupled waveguide 44 can increase the portion of the channelreturned to the gain medium 10. As a result, the couplers 42 should beconfigured to return enough of each channel to the gain medium 10 toachieve the desired level of lasing. The portion of a channel coupledinto the coupled waveguide 44 can be controlled by changing theseparation between a coupled waveguide 44 and the associated channelwaveguide 40. For instance, reducing the distance between a coupledwaveguide 44 and the associated channel waveguide 40 increases theportion of the channel that enters the coupled waveguide 44. In someinstances, the portion of a channel coupled into the coupled waveguide44 can also be controlled by changing the length for which the channelwaveguide 40 and the coupled waveguide 44 are close enough to each sharethe channel. In some instances, increasing this length can increase theportion of the channel that is coupled into the coupled waveguide 44.

A portion of a channel traveling through a coupler 42 can be returned tothe gain medium 10 as long as the channel is optically coupled into thecoupled waveguide 44. The region of the channel waveguide 40 where achannel traveling along the channel waveguide 40 is no longer coupledinto the coupled waveguide 44 serves as a port through which the channelexits the laser cavity. For instance, the lines labeled P in FIG. 2 canindicate where the channels exit the laser cavity through the port.

The multi-channel device of FIG. 2 employs a mach-zehnderinterferometers 50 as the modulators 20 of FIG. 1A. The mach-zehnderinterferometer 50 includes a first branch waveguide 52 and a secondbranch waveguide 53. A portion of a channel traveling along a channelwaveguide 40 enters the first branch waveguide 52 and another portion ofthe channel enters the second branch waveguide 53. The first branchwaveguide 52 and the second branch waveguide 53 join together at amodulated waveguide 54. The second branch waveguide 53 includes a phasemodulator 56. The phase modulator 56 can be employed to tune a phasedifferential between the portion of the channel in the first branchwaveguide and the portion of the channel in the second branch waveguidewhen they are joined at the modulated waveguide 54. Accordingly, themach-zehnder interferometer 50 can operate as an intensity modulator.

The multi-channel device of FIG. 2 employs an echelle grating 58 as themultiplexer 22 of FIG. 1A. The echelle grating 58 includes a free-spaceregion 60 and a reflecting surface 62. The modulated channels from themodulated waveguides 54 enter the free space region 60 of the echellegrating 58. The path of the light through the echelle grating 58 isillustrated as dashed lines in FIG. 2 in order to distinguish the lightfrom other features of the multi-channel device. The channels traveldifferent paths through the free space region 60 to the reflectingsurface 62 where they are reflected. The details of the reflectingsurface 62 are not shown in order to simplify the illustration. However,the reflecting surface of echelle grating 58 includes a plurality ofstepped reflecting surfaces. The reflecting surface causes the channelsto be combined as they travel away from the reflecting surface.Accordingly, the echelle grating 58 multiplexes the beam into amulti-channel beam traveling away from the reflecting surface. Thechannels are each received on an output waveguide 63.

As noted above, the multi-channel device can optionally opticalattenuators positioned in the laser cavity. FIG. 2 illustrates anoptical attenuator 23 positioned along each channel waveguide.Accordingly, an optical attenuator can be employed to attenuate achannel traveling along a channel waveguide. The optical attenuators arepositioned in the laser cavity. For instance, the optical attenuatorsare each positioned optically between the demultiplexer and the secondreflectors.

As noted above, the multi-channel device can optionally include anoptical amplifier 25 configured to amplify the modulated optical signalsafter they are multiplexed at the multiplexer 22. FIG. 2 illustrates anoptical amplifier 25 positioned along the output waveguide 63.Accordingly, the amplifier can be configured to amplify the modulatedoptical signals after they are multiplexed at the multiplexer 22.

The optical attenuators 23 can be positioned such that each opticalattenuator is configured to attenuate the intensity of one of thechannels. For instance, each optical attenuator can be positionedoptically between the demultiplexer 12 and the partial return device 16for a channel to be attenuated by the optical attenuator. One or more ofthe optical attenuators can be tunable. In one example, all of theoptical attenuators are tunable. Tunability of the optical attenutorspermits balancing of the power of the different channels to compensatefor loss or gain bias inside the laser cavity. In general, the channelshaving the highest and lowest wavelengths (λ₁ and λ₄ in FIG. 1) to seelower gains and higher losses. The attenuators can be employed toincrease the loss for the channels with the central wavelengths (λ₂ andλ₃ in FIG. 1). Placement of the optical attenuators in the lasing cavitypermits a redistribution of power to take place in the laser cavity suchthat losses introduced to the channels with the central wavelengthscauses a redistribution of gain medium power to the channels having thehighest and lowest wavelengths. As a result, a higher average power canbe achieved. The redistribution of power may not be achieved whenoptical attenuators are positioned outside of the laser cavity.

The multi-channel device can optionally include an optical amplifier 25configured to amplify the modulated optical signals after they aremultiplexed at the multiplexer 22. As a result, the optical amplifiercan concurrently amplify all of the channels.

Although the multi-channel device of FIG. 2 illustrates the power source18 included in the multi-channel device, the power source 18 can beseparate from the multi-channel device and the multi-channel device canbe configured to be coupled with the power source 18.

The multi-channel device of FIG. 2 can be built into a variety ofoptical component platforms. Suitable optical component platformsinclude, but are not limited to, a silicon-on-insulator platform. FIG.3A through FIG. 6F illustrate the various components of themulti-channel device illustrated in FIG. 2A on a silicon-on-insulatorplatform. Accordingly, the components illustrated in FIG. 3A throughFIG. 6F can be arranged on a silicon-on-insulator according to FIG. 2A.As a result, the components illustrated in FIG. 3A through FIG. 6F canbe combined so as to form a multi-channel device.

FIG. 3A through FIG. 3D illustrate a portion of a multi-channel devicehaving an interface between a cavity waveguide 32 and a gain medium chip24. The multi-channel device is constructed on a silicon-on-insulatorwafer. FIG. 3A is a topview of the multi-channel device. FIG. 3B is across section of the multi-channel device shown in FIG. 3A taken alongthe line labeled B. The line labeled B extends through the cavitywaveguide 32 disclosed in FIG. 2. Accordingly, FIG. 3B is a crosssection of the cavity waveguide 32. The silicon-on-insulator waferincludes a silica layer 64 between a silicon substrate 66 and a siliconslab 68. Trenches 70 in the silicon slab 68 define a ridge 72. The ridge72 and the silica layer 64 define a light signal-carrying region wherethe light beam is constrained. For instance, the reduced index ofrefraction of the silica relative to the silicon prevents the light beamfrom entering the substrate from the silicon. The other waveguides onthe multi-channel device have a structure similar to the structure shownin FIG. 3B although they can have different dimensions. For instance,the cavity waveguide 32, the channel waveguides 40, the coupledwaveguide 44, the branch waveguides, the modulated waveguide 54, and theoutput waveguide 63 can each have a structure as illustrated in FIG. 3B.

FIG. 3C is a cross section of the multi-channel device shown in FIG. 3Ataken along a line extending between the brackets labeled C in FIG. 3A.FIG. 3D is a cross section of the multi-channel device shown in FIG. 3Ataken along a line extending between the brackets labeled D in FIG. 3A.A first recess 71 extends into through the silicon slab 68 and thesilica layer 64. A second recess 72 extends into the bottom of the firstrecess 71 such that the silicon substrate 66 forms shelves 73 in thebottom of the second recess 72. A first conducting layer 75 ispositioned in the bottom of the second recess 72. A first conductor 76on the silicon slab 68 is in electrical communication with the firstconducting layer 75. A second conductor 77 on the silicon slab 68 ispositioned adjacent to the first recess 71.

The gain medium chip 24 is positioned in the first recess 71 and restson the shelves 73. The gain medium chip 24 includes a gain medium 10.Suitable materials for the gain medium 10 include, but are not limitedto a, an InP layer. A second conducting layer 78 is positioned on thegain medium 10. A third conductor 79 provides electrical communicationbetween the second conducting layer 78 and the second conductor 77.

Three ridges extending into the second recess 72. The outer-most ridgeshave a passivation layer. The central ridge is in electricalcommunication with the first conducting layer 75. The electricalcommunication between the central ridge and the first conductor 76 canbe achieved through a conducting medium 80 such as solder. Since thefirst conductor 76 is in electrical communication with the firstconducting layer 75, the first conductor 76 is in electricalcommunication with the central ridge.

The beam of light can be generated from the gain medium 10 by causing anelectrical current to flow through the gain medium 10. The electricalcurrent can be generated by applying a potential difference between thefirst conductor 76 and the second conductor 77. The potential differencecan be provided by the power source 18 illustrated in FIG. 2. The powersource 18 can be included on the multi-channel device or can be separatefrom the multi-channel device and the multi-channel device can beconfigured to be electrically coupled with the power source 18.

The gain medium chip 24 includes a reflecting surface on the gain medium10. The reflecting surface serves as the first reflector 14 of FIG. 1 oras the reflecting surface of FIG. 2. Suitable reflecting surfacesinclude a layer of metal on the layer of gain medium 10. The side of thegain medium 10 opposite the reflecting surface includes ananti-reflective coating 29. The beam of light exits the gain medium 10through the anti-reflective coating 29.

As is evident from FIG. 3A, the facet 81 for the cavity waveguide 32 canbe angled at less than ninety degrees relative to the direction ofpropagation in the cavity waveguide 32. Angling the facet 81 at lessthan ninety degrees can cause light signals reflected at the facet 81 tobe reflected out of the waveguide and can accordingly reduce issuesassociated with back reflection.

The trenches 70 for the waveguides can be formed using traditionalintegrated circuit manufacturing masking and etching steps. The firstrecess 71 can be formed in a different mask and etch. Further, thesecond recess 72 can be formed in another mask and etch steps. The firstconducting layer 75, the first conductor 76, and the second conductor 77can be formed using traditional integrated circuit manufacturingtechniques for forming metal traces on substrates.

Suitable gain medium chips 24 include, but are not limited to, InPchips. The electrical communication between the second conducting layer78 and the second conductor 77 can be achieved using traditionaltechniques such as wire bonding. The electrical communication betweenthe central ridge and the first conductor 76 can be achieved throughtraditional techniques such as solder bonding.

The portion of the multi-channel device illustrated in FIG. 3A throughFIG. 3D is suitable for use with an electrical power source, however,the illustrations in FIG. 3A through FIG. 3D can be adapted for use witha light source that serves as a power source. The light source can beincluded on the multi-channel device or can be separate from themulti-channel device.

FIG. 4A and FIG. 4B illustrate a portion of a multi-channel devicehaving an interface between an echelle grating 34, a cavity waveguide32, and channel waveguides 40. The multi-channel device is formed on asilicon-on-insulator wafer. FIG. 4A is a topview of thesilicon-on-insulator wafer. FIG. 4B is a cross section taken along theline labeled B in FIG. 4A. As noted in FIG. 2, the echelle grating 34includes a free space region 36 and a reflecting surface 38. The beamfrom the cavity waveguide 32 enters the free space region 36 of theechelle grating 34. The path of the light through the echelle grating 34is illustrated as dashed lines in FIG. 4A in order to distinguish thelight from other features. The beam travels through the free spaceregion 36 and is reflected off of the reflecting surface 38. The echellegrating 34 demultiplexes the beam into the individual channels travelingaway from the reflecting surface 38.

A reflecting recess 81 extends through the silicon slab 68 to the silicalayer 64 and can extend to into or through the silica layer 64. A sideof the reflecting recess 81 serves as the reflecting surface 38. Theside of the reflecting recess 81 can optionally include a reflectingmaterial 83 to enhance reflection of light from the free space region36. Suitable reflecting materials 83 include a layer of metal. Thereflecting recess 81 can be filled with air or can optionally be filledwith a cladding material such as silica.

The channel waveguides 40 and the cavity waveguide 32 shown in FIG. 4Ahave the same general structure as the cavity waveguide 32 shown in FIG.3A. For instance, the channel waveguides 40 and the cavity waveguide 32can have the cross section illustrated in FIG. 3B. As is evident in FIG.4A, the channel waveguides 40 and the cavity waveguide 32 do notterminate at a facet but instead open up into the free space 36 regionof the echelle grating 34.

The trenches 70 for the channel waveguides 40 can be formed concurrentlywith the trenches 70 for the cavity waveguide 32. The reflecting recess81 can optionally be masked and etched concurrently with the firstrecess 71 for the gain medium chip 24 or can be masked and etched in adifferent step. The reflecting material 83 can be formed in thereflector recess 84 using traditional integrated circuit manufacturingtechniques.

The echelle grating 34 of FIG. 4A and FIG. 4B can serve as themultiplexer 22 of FIG. 1A and/or FIG. 1B and/or as the echelle grating34 of FIG. 2. For instance, the channel waveguides 40 of FIG. 4A canserve as the modulated waveguides 54 and the cavity waveguide 32 canserve as the output waveguide 63. The modulated channels travel from themodulated waveguide 54, through the echelle grating 34, to the outputwaveguide 63. The output waveguide 63 can have the same generalstructure as the channel waveguide 40, the first branch waveguide 52,the second branch waveguide 53, the modulated waveguides 54, and/or thecavity waveguide 32. For instance, the output waveguide 63 can have thecross section illustrated in FIG. 3B. The trenches 70 for the outputwaveguide 63 can be masked and etched concurrently with the trenches 70for the channel waveguide 40, the first branch waveguide 52, the secondbranch waveguide 53, the modulated waveguides 54, and/or the cavitywaveguide 32.

FIG. 5A through FIG. 5C illustrate the portion of a multi-channel devicehaving an optical coupler 42 that can serve as a partial return device.The multi-channel device is constructed on a silicon-on-insulator wafer.Each of the optical couplers 42 illustrated in FIG. 2 can be constructedaccording to FIG. 5A through FIG. 5C. FIG. 5A is a topview of themulti-channel device. The coupler 42 includes a channel waveguide 40 anda coupled waveguide 44. FIG. 5B is a cross section of the coupledwaveguide and the channel waveguide 40 taken along the line labeled B inFIG. 5A. The ridge 72 for the coupled waveguide 44 is close enough tothe ridge 72 for the channel waveguide 40 for the channel waveguide 40and the coupled waveguide 44 to physically share a channel as it travelsalong the channel waveguide 40. Accordingly, a portion of the channeltransfers into the coupled waveguide 44 before the ridge 72 of thecoupled waveguide 44 and the ridge 72 of the channel waveguide 40physically separate as shown in FIG. 5A.

FIG. 5C is a cross section of the coupled waveguide 44 taken along aline between the brackets labeled C in FIG. 5A. The coupled waveguide 44terminates at a reflector recess extending through the silicon slab 68to or into the silica layer 64. A side of the reflector recess 84 servesas a reflecting surface. The reflecting surface serves as the reflectingdevice 46 disclosed in the context of FIG. 2. The side of the reflectorrecess 84 can optionally include a reflecting material 85 to enhancereflection of light back through the coupled waveguide 44. Suitablereflecting materials 85 include a layer of metal. The reflector recess84 can be filled with air or can optionally be filled with a claddingmaterial such as silica.

The trenches 70 for the channel waveguides 40 and the coupled waveguides44 can be formed concurrently with the trenches 70 for the cavitywaveguide 32. The reflector recess 84 can optionally be masked andetched concurrently with the first recess 71 for the gain medium chip 24and/or with the reflecting recess 81 for the echelle grating 34.Alternately, the reflector recess 84 can be masked and etched in adifferent step. The reflecting material 85 can be formed in thereflector recess 84 using traditional integrated circuit manufacturingtechniques.

FIG. 6A through FIG. 6E illustrate a portion of a multi-channel devicehaving a mach-zehnder interferometer configured to operate as anintensity modulator. The multi-channel device is constructed on asilicon-on-insulator wafer. FIG. 6A is a topview of the mach-zehnderinterferometer. The mach-zehnder interferometer includes a channelwaveguide 40 that branches into a first branch waveguide 52 and a secondbranch waveguide 53. The first branch waveguide 52 re-joins the secondbranch waveguide 53 at a modulated waveguide 54.

The channel waveguide 40, the first branch waveguide 52, the secondbranch waveguide 53 and the modulated waveguide 54 have the same generalstructure as the cavity waveguide 32 shown in FIG. 3A. For instance, thechannel waveguide 40, the first branch waveguide 52, the second branchwaveguide 53 and the modulated waveguide 54 can have the cross sectionillustrated in FIG. 3B. The trenches 70 for the channel waveguide 40,the first branch waveguide 52, the second branch waveguide 53 and/or themodulated waveguide 54 can be masked and etched concurrently with thetrenches 70 for the cavity waveguide 32.

A phase modulator 56 is positioned along the second branch waveguide 53.FIG. 6B is a topview of a phase modulator 56 that is suitable for usewith the mach-zehnder interferometer of FIG. 6A. FIG. 6C is a crosssection of the phase modulator 56 shown in FIG. 6B taken along the linelabeled C in FIG. 6B. A filler 122 such as a solid or a gas ispositioned in the trenches 70 that define the second branch waveguide53. The filler 122 has an index of refraction lower than the index ofrefraction of the silicon in order to constrain the light signals withinthe ridge 72. The filler can also provide electrical isolation betweendifferent regions of the phase modulator. For instance, the filler canprovide electrical isolation between the first doped region and thesecond doped region, which are discussed in more detail below. Asuitable filler 122 includes, but is not limited to, silica. A vacuumcan also serve as a suitable filler 122.

An insulating layer 124 is positioned on the light-transmitting medium10 and the filler 122. The insulating layer is illustrated in FIG. 6Cbut is not illustrated in FIG. 6B to simplify the illustration. Theinsulating layer 124 can provide electrical insulation and/or opticalconfinement. A suitable insulating layer 124 includes, but is notlimited to, low K dielectrics such as silica, and/or silicon nitride. Inone example, the insulating layer 124 includes a silicon nitride andoxide bi-layer over silicon.

An upper layer 125 is positioned on the insulating layer 124. The upperlayer 125 is illustrated in FIG. 6C but is not illustrated in FIG. 6B tosimplify the illustration. The upper layer 125 can serve to reduce orprevent capacitive coupling between different components in the device.For instance, the upper layer 125 can prevent or reduce capacitivecoupling between a first conducting member 126 and a second conductingmember 128 that are disclosed in more detail below. A suitable upperlayer includes, but is not limited to, low K dielectrics such as silica.

The phase modulator includes a first conducting member 126 and a secondconducting member 128 as is evident in both FIG. 6B and FIG. 6C. In FIG.6B, the first conducting member 126 and the second conducting member 128are illustrated by dashed lines and are shown as transparent to permit aview of the underlying features. The first conducting member 126 and thesecond conducting member 128 can serve as electrodes but more preferablyserve as transmission lines. Suitable materials for the first conductingmember 126 include, but are not limited to, aluminum, copper and/ortheir alloys. Suitable materials for the second conducting member 128include, but are not limited to, aluminum, copper and/or their alloys.

A first electrical connector 130 provides electrical communicationbetween the first member 126 and a contact portion of the silicon slab68 located adjacent to the waveguide and spaced apart from thewaveguide. Second electrical connectors 132 provide electricalcommunication between contacts 134 at the top of the ridge 72 and thesecond member 128. The first electrical connector 130 and the secondelectrical connectors 132 are illustrated in FIG. 6C but are notillustrated in FIG. 6B to simplify the illustration. The firstelectrical connectors, the second electrical connectors and the contactsprovide electrical connections between electronics and the optics.Suitable materials for the first electrical connector 130 include, butare not limited to, tungsten, aluminum, copper and/or their alloys.Suitable materials for the second electrical connector 132 include, butare not limited to, tungsten, aluminum, copper and/or their alloys.Suitable materials for the contacts 134 include, but are not limited to,Al—Si alloys, Ti silicide, and Co silicide.

In some instances, the contacts 134 are a doped non-metal such as dopedsilicon or doped polysilicon. Doped polysilicon can provide the requiredelectrical conduction but can have about two orders of magnitude fewercarriers than the metal. Because increased carrier content is associatedwith increased light absorption, contacts 134 constructed from dopedsilicon can be associated with reduced levels of optical loss relativeto metals. As a result, contacts 134 constructed of doped silicon orpolysilicon may be desired when low levels of optical loss are desired.When the contacts 134 are made of polysilicon, a suitable concentrationof the dopant includes, but is not limited to, concentrations of about10¹⁸/cm³ to 2×10²¹/cm³ or 10¹⁹/cm³ to 2×10²⁰/cm³.

The silicon is doped so as to have a first doped region 136 and a seconddoped region 138. When the first doped region 136 is an n-type region,the second dope region is a p-type region. When the first doped region136 is a p-type region, the second dope region is an n-type region. Insome instances, the first doped region is preferably an n-type regionand the second doped region is preferably a p-type region. For instance,certain fabrication techniques may permit easier formation of a p-typeregion deeper in the light transmitting medium that an n-type region.When the contacts 134 are formed of a doped non-metal, the non-metal isdoped with the same type of dopant as the first doped region 136 but canbe at a higher dopant concentration than the first doped region 136.

The first doped region 136 and the second doped region 138 arepositioned sufficiently close to one another that a depletion region 140forms between the n-type region and the p-type region when a bias is notapplied to the phase modulator. For instance, FIG. 1B illustrates then-type region in contact with the p-type region. Contact between then-type region and the p-type region may not be necessary although it canincrease the efficiency of the modulator. The resulting interface issubstantially parallel to the top of the ridge 72 and/or the siliconsubstrate 66 and is positioned in the ridge 72.

The depletion region 140 results from a migration of carriers betweenthe n-type region and the p-type region until a potential forms thatprevents additional migration. This migration results in a lack ofcarriers in the depletion region. For instance, the depletion region 140has a carrier concentration of less than about 1×10¹⁵/cm³. The n-typeregion and a p-type region are positioned so the depletion region 40 ispositioned in the light signal-carrying region of the waveguide. Forinstance, FIG. 1C illustrates the depletion region 140 that forms fromthe doped region configuration illustrated in FIG. 1B. A suitableconcentration of carriers in the p-type region includes values greaterthan 1×10¹⁵/cm³, 1×10⁶/cm³, 3.5×10¹⁶/cm³, or 5.0×10¹⁷/cm³. A suitablevalue for the concentration of carriers in the n-type region includesvalues greater than 1×10¹⁵/cm³, 2×10¹⁶, 5×10⁶, and 1×10⁸ cm⁻³.

A secondary doped region 144 is formed at the contact portion of thesilicon slab 68. The secondary doped region 144 can contact the adjacentdoped region and can include the same type of dopant as the adjacentdoped region. For instance, in FIG. 6C, the underlying doped region isthe second doped region 138. Accordingly, when the phase modulator isconstructed as illustrated in FIG. 6C, the secondary doped region 144can contact the second doped region and has a dopant type that is thesame as the second doped region 138. The secondary doped region 144 canhave a higher dopant concentration than the adjacent doped region. Forinstance, the dopant concentration in the secondary doped region 144 canbe more than 10 times the dopant concentration in the adjacent dopedregion or more than 1000 times the dopant concentration in the adjacentdoped region. The elevated dopant concentration reduces the contactresistance of the phase modulator and accordingly provides an increasedmodulation speed. Suitable concentrations for the dopant in thesecondary doped region 144 include, but are not limited to,concentrations greater than 1×10⁸/cm³, 1×10⁹/cm³, 5×10¹⁹/cm³,1×10²⁰/cm³. Increasing the dopant concentration can increase the amountof optical loss. As a result, the secondary doped region 144 ispositioned remote from the light signal-carrying region in order toreduce optical loss resulting from the increased dopant concentration.For instance, the secondary doped region 144 is positioned on a portionof the silicon slab 68 adjacent to the trench 70. This location canreduce interaction between a light signal in the waveguide and thesecondary doped region 144. In some instances, the secondary dopedregion 144 can be positioned in the trench 70 or in the bottom of thetrench 70.

The first member 126 and the second member 128 are connected toelectronics (not shown) that can apply a bias between the firstconducting member 126 and the second conducting member 128. Accordingly,a bias is formed between the top of the ridge 72 and the contact portionof the silicon slab 68. The bias can be a reverse bias. Changing thelevel of bias changes the size and/or shape of the depletion region. Forinstance, increasing the reverse bias can increase the size of thedepletion region. As an example, FIG. 6E illustrates the depletionregion of FIG. 6D after an increased reverse bias has been applied tothe phase modulator. FIG. 6C, FIG. 6D and FIG. 6E illustrate the firstdoped region and the second doped region occupying the entire lightsignal carrying region. This arrangement can provide an increasedpotential tuning efficiency.

The depletion region 140 has a different index of refraction than thelight transmitting region located adjacent to the depletion region. Forinstance, when the light-transmitting medium 110 is silicon, thedepletion region 140 has a higher index of refraction than that of thesurrounding silicon. As a result, the depletion region 140 slows thelight signal as the light signal travels through the depletion region.As a result, increasing the size of the depletion region 140 furtherslows the speed at which the light signal travels through the waveguide.Accordingly, the speed of the light signal through the waveguide can betuned by tuning the bias level. Additionally, because this phase tuningis based on tuning of the depletion region, tuning of the phasemodulator does not involve carrier re-combination. Carrier recombinationis on the order of 1000 times slower than changes in the depletionregion. Accordingly, the phase modulator can be on the order of 1000 to10000 times faster than phase modulators that require carrierrecombination.

A forward bias can be applied to the phase modulator. The forward biaswill shrink the size of the depletion region. Accordingly, increasingthe forward bias can accelerate the light signal. However, once theforward bias rises above a threshold, the forward bias can result incurrent flow that requires recombination as the forward bias dropstoward the threshold. Because tuning that requires recombination isslower than tuning of the depletion region, it may not be desirable touse the forward bias above levels where significant current flow occurs.

The concentration of the dopants in the doped regions influences theperformance of the phase modulator. For instance, the dopants can causelight absorption. As a result, increasing the dopant level can causeundesirably high levels of optical loss. Decreasing the dopant level canreduce the tuning efficiency by requiring a higher bias level to achievethe same level of phase modulation. As a result, when the dopant levelis reduced, the length of the phase modulator must be increased toprovide the desired level of phase modulation for a give bias level.Suitable dopants for the n-type region include, but are not limited to,phosphorus and/or arsenic. Suitable dopants for the p-type regionsinclude, but are not limited to, boron.

Although FIG. 6C illustrates the interface between the first dopedregion 136 and the second doped region 138 as being positioned in theridge 72, first doped region 136 and the second doped region 138 can beconstructed so the interface is below the ridge 72. In these instances,the doped region in the ridge 72 and the secondary doped region 144 maybe the same type of doped region. For instance, the doped region in theridge 72 and the secondary doped region 144 may both be an n-type regionor they may both be a p-type region.

In some instances, it is desirable for an intensity modulator such as aMach-Zehnder interferometer to provide intensity modulation on the orderof 10 to 40 Gbit/s with low levels of optical loss. Accordingly, thehigh-speed features of the phase modulator can be important when thephase modulator is employed for intensity modulation. Additionally, thelow optical loss features of the phase modulator can also becomedesirable when the phase modulator is employed for intensity modulation.

In some instances, it may be preferable for the modulator to be a phasemodulator rather than an intensity modulator. For instance, FIG. 6Fillustrates the phase modulator of FIG. 6B through FIG. 6E substitutedfor the intensity modulator illustrated in FIG. 6A. In FIG. 6F, thefirst branch waveguide 52 and the second branch waveguide 53 areeliminated and the channel waveguide 40 is connected directly to themodulated waveguide 54. The phase modulator is positioned at theintersection of the channel waveguide 40 and the modulated waveguide 54.

Additional information regarding the structure, fabrication, andoperation of a high speed intensity modulator are provided in U.S.patent application Ser. No. 11/146,898, filed on Jun. 7, 2005, entitled“High Speed Optical Phase Modulator” and in U.S. patent application Ser.No. 11/147,403, filed on Jun. 7, 2005, entitled “High Speed OpticalIntensity Modulator,” each of which is incorporated herein in itsentirety. Additionally, U.S. patent application Ser. Nos. 11/146,898 and11/147,403 provide additional embodiments for phase and intensitymodulators that can be employed as modulators in the multi-channeldevice disclosed above.

The filler 122, the upper layer 125, and the insulating layer 124 thatare present in FIG. 6C are not shown in FIG. 3A through FIG. 5C. Thefiller 122, the upper layer 125, and the insulating layer 124 can belocalized to the modulator. Alternately, the upper layer 125 and theinsulating layer 124 can be positioned on the exposed silicon slab 68and the filler 122 can be positioned in the trenches 70 of FIG. 3Athrough FIG. 5C.

FIG. 7 is a cross section of a waveguide on a silicon-on-insulatorwafer. As noted above, the cavity waveguide 32, the channel waveguides40, the coupled waveguide 44, the first branch waveguides 52, the secondbranch waveguides 53, the modulated waveguides 54 can each have astructure according to FIG. 7. The waveguide has a width labeled W. Thewidth is the width of the ridge at the top of the ridge. The waveguidealso has a thickness labeled H. The thickness is the thickness of thesilicon where the light signal is carried. For instance, the thicknessextends from the top of the silica layer 64 to the top of the ridge. Thecross sectional area of the waveguide is equal to the width, W,multiplied by the thickness, H. The light signal may extend outside ofthis cross sectional area as the light signal travels through thewaveguide. The intensity modulator and the phase modulator illustratedin FIG. 6A through FIG. 6E is more efficient and faster as the waveguidecross-section decreases. In one example, the first branch waveguide 52and the second branch waveguide 53 have a cross sectional area of about1 μm².

An echelle grating such as the echelle grating 34 illustrated in FIG. 2has more loss as the thickness of the echelle grating increases. Thethickness is the thickness of the silicon where the light signal iscarried. Accordingly, the thickness is the can be from the top of thesilica to the top of the silicon slab 68 in the free space region.

In some instances, the cavity waveguide 32, the channel waveguides 40,the first branch waveguides 52, the second branch waveguides 53, themodulated waveguides 54 all have about the same cross-sectionaldimensions but the output waveguide 63 have larger cross-sectionaldimensions. This arrangement permits the modulators and echelle gratingsto have the dimensions that are desired for efficient operation of theechelle grating and the modulators while permitting the output waveguide63 to have the dimensions desired for other applications.

In some instances, waveguide tapers can be employed to vary thecross-sectional dimensions of the waveguides. For instance, FIG. 8A is atopview of the interface between the echelle grating, the cavitywaveguide 32, and the channel waveguides 40 on a silicon-on-insulatorwafer. The channel waveguides 40 include a horizontal taper 150. Thetaper 150 can reduce the cross-sectional dimension of the channelwaveguides 40 to dimensions that are suitable for efficient operation ofthe modulators. Each taper 150 can have a horizontal taper 150 without avertical taper, or can have a vertical taper and horizontal taper.Although FIG. 8A illustrates the tapers 150 as having a horizontaltaper, the tapers can have vertical taper without horizontal taper.

In addition to the tapers illustrated in FIG. 8A or as an alternative tothe tapers illustrated in FIG. 8A, tapers 150 can be employed inconjunction with the modulator. For instance, FIG. 8B illustrates themodulator positioned between tapers 150. The direction of the channelsthrough the modulator and the tapers is illustrated by the arrow labeledA. Each taper can have a horizontal taper without a vertical taper, orcan have a vertical taper and horizontal taper. Although FIG. 8Aillustrates the tapers as having a horizontal taper, the tapers can havevertical taper without horizontal taper. The taper can reduce thecross-sectional dimension of the channel waveguides 40 to dimensionsthat are suitable for efficient operation of the modulators. In someinstances, one of the tapers is not employed. For instance, the taperlabeled P need not be employed. In the instances where the taperslabeled P are not employed, the cross-sectional dimensions of the outputwaveguide 63 can be larger than the cross-sectional dimensions of themodulated waveguide 54. As a result, the multiplexer provides theexpansion of the cross-sectional dimensions that was previously providedby the taper labeled P.

The modulator illustrated in FIG. 8B can be the modulator of FIG. 1. Forinstance, the modulator can be the intensity modulator of FIG. 6A, thephase modulator of FIG. 6B, or the phase modulator of FIG. 6F. Further,the modulator can be a phase modulator of FIG. 6B included in anintensity modulator of FIG. 6A.

Suitable structures for the tapers and methods for fabricating thetapers are provided in U.S. patent application Ser. No. 10/345,709,filed on Jan. 15, 2003, entitled “Controlled Selectivity Etch for Usewith Optical Component Fabrication,” and incorporated herein in itsentirety. The disclosed structures and methods can be employed for thetapers illustrated in FIG. 8A and FIG. 8B.

As noted above in the context of FIG. 1B, the multi-channel device canbe constructed such that one or more multiplexers multiplexes channelsfrom two or more laser cavities. When one or more multiplexers multiplexthe channels from different laser cavities, the different laser cavitiesand the multiplexer can be included on the same multi-channel device.For instance, the different laser cavities and the multiplexer can beincluded on the same wafer. For instance, the different laser cavitiesand the multiplexer can be included on a silicon-on-insulator wafer.

Although the multi-channel device is disclosed in the context of asilicon-on-insulator wafer, the multi-channel device can be built intoother platforms. Additionally, the multi-channel device is disclosed inthe context of ridge waveguides. However, the multi-channel device canbe constructed using other waveguides including, but not limited to,buried channel waveguides 40.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

1. A multi-channel optical device, comprising: a laser cavity thatincludes a gain medium configured to generate a multi-channel lightbeam, a demultiplexer configured to demultiplex the multi-channel lightbeam into a plurality of channels; partial return devices that eachreceives one of the channels, each of the partial return devicesconfigured to transmit a portion of the received channel such that thetransmitted portion exits the laser cavity, and each of the partialreturn devices configured to return a portion of the received channel;and wherein the gain medium receives the returned portions of thechannels.
 2. The device of claim 1, further comprising: a plurality ofchannel waveguides that are each configured to receive one of thechannels, each channel waveguide including one of the ports.
 3. Thedevice of claim 2, wherein each channel waveguide is included in anoptical coupler, each optical coupler including a coupled waveguideoptically coupled with one of the channel waveguides such that a portionof a channel traveling along the coupled channel waveguide enters thecoupled waveguide.
 4. The device of claim 3, wherein the coupledwaveguides include a reflector configured to reflect at least a portionof the channel that enters the coupled waveguide back onto the coupledchannel waveguide.
 5. The device of claim 4, wherein the reflector ispositioned at a terminal end of the coupled waveguide.
 6. The device ofclaim 5, wherein the reflector includes a reflecting material positionedat the terminal end of the coupled waveguide.
 7. The device of claim 3,further comprising: a plurality of modulators that are each configuredto modulate a channel traveling along one of the channel waveguidesafter the channel has passed through the optical coupler, modulation ofeach channel resulting in a modulated channel.
 8. The device of claim 7,wherein each modulator includes a phase modulator configured to modulatea phase of a channel traveling along one of the channel waveguides. 9.The device of claim 8, wherein the phase modulators each include ann-type region having a proximity to a p-type region that causes adepletion region to form when a bias is not applied to the phasemodulator, the depletion region being at least partially positioned in alight signal carrying region of the waveguide.
 10. The device of claim7, wherein each modulator includes a mach-zehnder interferometer with aplurality of branch waveguides and a phase modulator configured tomodulate a phase of a channel traveling through one of the branchwaveguides.
 11. The device of claim 7, wherein each modulator includes aphase modulator positioned along a branch waveguide of a mach-zehnderinterferometer, the phase modulator including an n-type region having aproximity to a p-type region that causes a depletion region to form whena bias is not applied to the modulator, the depletion region being atleast partially positioned in a light signal carrying region of thewaveguide.
 12. The device of claim 7, further comprising: a multiplexerconfigured to receive a modulated channel from each of the modulatorsand to multiplex the modulated channels into a beam having a pluralityof modulated channels.
 13. A multi-channel optical device, comprising: ademultiplexer in a laser cavity, the demultiplexer configured todemultiplex a multi-channel light beam into a plurality of channels; achannel waveguide configured to receive one of the channels; an opticalcoupler including a coupled waveguide optically coupled with the channelwaveguide such that a portion of the channel received by the channelwaveguide enters the coupled waveguide; and a reflector configured toreflect at least a portion of the channel that entered the coupledwaveguide back into the channel waveguide such that the reflectedportion of the channel travels through the channel waveguide back to thedemultiplexer.
 14. The device of claim 13, wherein the channel waveguideis one of a plurality of channel waveguides, each channel waveguideconfigured to receive a different one of the channels and also beingincluded in an optical coupler.
 15. The device of claim 13, wherein thereflector is positioned at a terminal end of the coupled waveguide. 16.The device of claim 13, wherein the reflector includes a reflectingmaterial positioned at the terminal end of the coupled waveguide. 17.The device of claim 13, further comprising: a modulator configured tomodulate the channel received by the channel waveguide after the channelhas passed through the optical coupler, modulation of the channelresulting in a modulated channel.
 18. The device of claim 17, whereinthe modulator includes a phase modulator configured to modulate thechannel received by the channel waveguide.
 19. The device of claim 18,wherein the phase modulator includes an n-type region having a proximityto a p-type region that causes a depletion region to form when a bias isnot applied to the phase modulator, the depletion region being at leastpartially positioned in a light signal carrying region of the waveguide.20. The device of claim 17, wherein the modulator includes amach-zehnder interferometer with a plurality of branch waveguides and aphase modulator configured to modulate a phase of the channel receivedby the channel waveguide.
 21. The device of claim 17, wherein themodulator includes a phase modulator positioned along a branch waveguideof a mach-zehnder interferometer, the phase modulator including ann-type region having a proximity to a p-type region that causes adepletion region to form when a bias is not applied to the modulator,the depletion region being at least partially positioned in a lightsignal carrying region of the waveguide.
 22. The device of claim 17,further comprising: a multiplexer configured to receive the modulatedchannel from the modulator and to multiplex the modulated channel withmodulated channels from other modulators into a beam having a pluralityof modulated channels.
 23. A multi-channel optical device, comprising: ademultiplexer in a laser cavity, the demultiplexer configured todemultiplex a multi-channel light beam into a plurality of channels;optical attenuators in the laser cavity, each of the attenuatorsconfigured to receive one of the channels and to optically attenuate thereceived channel; and a plurality of modulators, each modulatorconfigured to modulate one of the channels after the channel has exitedthe laser cavity, modulation of each channel resulting in a modulatedchannel.
 24. The device of claim 23, further comprising: a plurality ofchannel waveguides that are each configured to receive one of thechannels, each channel waveguide including a port through which one ofthe channels exits the laser cavity.
 25. The device of claim 24, whereineach channel waveguide is included in an optical coupler, each opticalcoupler including a coupled waveguide optically coupled with one of thechannel waveguides such that a portion of a channel traveling along thecoupled channel waveguide enters the coupled waveguide.
 26. The deviceof claim 25, wherein the coupled waveguides include a reflectorconfigured to reflect at least a portion of the channel that such thatthe reflected portion of the channel travels through the channelwaveguide back to the demultiplexer.
 27. The device of claim 26, whereinthe reflector is positioned at a terminal end of the coupled waveguide.28. The device of claim 26, wherein the reflector includes a reflectingmaterial positioned at the terminal end of the coupled waveguide. 29.The device of claim 25, wherein the modulators are each configured toreceive one of the channels channel after the channel has passed throughthe optical coupler.
 30. The device of claim 23, wherein each modulatorincludes a phase modulator configured to modulate a phase of one of thechannels.
 31. The device of claim 30, wherein the phase modulators eachinclude an n-type region having a proximity to a p-type region thatcauses a depletion region to form when a bias is not applied to thephase modulator, the depletion region being at least partiallypositioned in a light signal carrying region of the waveguide.
 32. Thedevice of claim 23, wherein each modulator includes a mach-zehnderinterferometer with a plurality of branch waveguides and a phasemodulator configured to modulate a phase of a channel traveling throughone of the branch waveguides.
 33. The device of claim 32, wherein eachmodulator includes a phase modulator positioned along a branch waveguideof a mach-zehnder interferometer, the phase modulator including ann-type region having a proximity to a p-type region that causes adepletion region to form when a bias is not applied to the modulator,the depletion region being at least partially positioned in a lightsignal carrying region of the waveguide.
 34. The device of claim 23,further comprising: a multiplexer configured to muiltiplex the modulatedchannels into a beam having a plurality of modulated channels.
 35. Amulti-channel optical device, comprising: a demultiplexer in a lasercavity, the demultiplexer configured to demultiplex a multi-channellight beam into a plurality of channels; optical attenuators in thelaser cavity, each of the attenuators configured to receive one of thechannels and to optically attenuate the received channel; and amultiplexer configured to muiltiplex the channels after the channel exitthe laser cavity.
 36. A multi-channel optical device, comprising: ademultiplexer in a laser cavity, the demultiplexer configured todemultiplex a multi-channel light beam into a plurality of channels; aplurality of channel waveguides that are each configured to receive oneof the channels, each channel waveguide including a port through whichone of the channels exits the laser cavity; each channel waveguide beingincluded in an optical coupler, each optical coupler including a coupledwaveguide optically coupled with one of the channel waveguides such thata portion of a channel traveling along the coupled channel waveguideenters the coupled waveguide; each coupled waveguide includes areflector configured to reflect at least a portion of the channel thatenters the coupled waveguide back onto the coupled channel waveguidesuch that the reflected portion of the channel travels through thechannel waveguide back to the demultiplexer; a plurality of modulatorsthat are each configured to modulate a channel traveling along one ofthe channel waveguides after the channel has passed through one of theoptical couplers, modulation of each channel resulting in a modulatedchannel; and a multiplexer configured to receive a modulated channelfrom each of the modulators and to multiplex the modulated channels intoa beam having a plurality of modulated channels.
 37. The device of claim1, further comprising: optical attenuators in the laser cavity, each ofthe attenuators configured to receive one of the channels and tooptically attenuate the received channel.
 38. The device of claim 13,further comprising: optical attenuators in the laser cavity, each of theattenuators configured to receive one of the channels and to opticallyattenuate the received channel.